The invention relates to a method and apparatus for drying pasty, aqueous sludges, which result as wastes of industrial production and as by-products of municipal sewage treatment plants.
Pasty and aqueous sludges, in this context, have a solid matter content which generally exceeds 15% by weight. Normally, the solid matter proportion would be within the range of about 15% to 18% by weight. In many instances, industrial and municipal sludges are involved which have been subjected to a preliminary water withdrawal and which accumulate in large quantities. Recently, the removal of these sludges has become a great problem. Removal shall not only be made very quickly, but also with the smallest possible energy input, whereby valuable residues shall not be destroyed. If possible, the residues are to be recycled as raw materials into the economic process in order to save energy and raw materials.
Thus, the invention is concerned not only with a process for drying pasty, aqueous sludges, but in a more specific sense also with a process for producing certain raw materials from specific waste sludges of municipal or industrial treatment plants, such as sewage treatment plants.
Without any restriction, such production includes, for example the manufacture of solid fuels from aqueous oil sludges, the production of pigments from aqueous paint, varnish and metal oxide sludges, and the production of fertilizers from waste sludges containing nutritive, feed and/or faecal substances.
In the production of fertilizers, especially of biological compost, it may be advantageous to add to the starting sludges a certain percentage of specific organic waste sludges as a binding agent to improve the adhesion of the individual solid particles to each other. According to the invention, these binding agents may be particularly milk and/or molasses sludges available from the dairy and sugar industries.
Waste sludges mixed with such specific binding agent sludges, which are dried or concentrated according to the invention, may be easily processed further to form slab-shaped or sheet type products which may be stored in a particularly space-saving manner.
The structure or composition of the sludges may be of the most varied types. In particular the sludges may contain pressed sugar beet cossettes or pulp, whereby it is the objective to obtain dry, storable, pressed sugar beet cossettes to be used as animal feed.